Powders composed of coarse and fine particles are utilized in many industrial processes. Examples of powders include foods, pharmaceuticals, abrasives, pigments, plastics, magnetic coating materials and the like. The particles may range in diameter from less than one micrometer to one thousand micrometers. One known technique for particle size measurement utilizes a time-of-flight technique. An aerosol containing the particles to be measured is accelerated through a nozzle and is injected through two spaced-apart light beams in a vacuum chamber. As a particle passes through each light beam, light scattering occurs. A detector receives a scattered light pulse from the first beam and from the second beam for each particle. The time delay between pulses represents particle velocity, which is directly correlated to particle size. The output of such an instrument is typically a distribution of the number of particles measured at each particle size over a range of sizes. An example of such a system is the Aerosizer.RTM. particle sizing system manufactured and sold by Amherst Process Instruments, Inc.
A critical component of time-of-flight particle size measurement systems is the powder disperser which delivers a fully-suspended, evenly-dispersed, primary particle aerosol to the particle measurement zone. Samples of particles to be measured are often in the form of a dry powder sample. The powder sample is an agglomerated form of primary particles which are usually in clusters when in a cohesive state, as opposed to individual particles in a free flowing state. These clusters are due to several types of attraction mechanisms, working in part or together: electrostatic or Van Der Wall attraction, "thin film" liquid surface attraction, and mechanical surface geometry interlocking.
In order to accurately measure the particles using the time-of-flight technique, the clusters must be deagglomerated into individual particles and entrained into a gas stream for presentation to the measurement zone in a random, one at a time fashion. If agglomerated particles are not fully dispersed prior to entering the acceleration nozzle, they will be obliterated within the measurement zone. The result of this mal-dispersion condition is the rapid particulate coating of the source and detection optics. The fine powder build-up on these surfaces degrades instrument performance, thus demanding frequent optics cleaning and maintenance.
A powder disperser for an aerodynamic particle sizing system is disclosed in U.S. Pat. No. 4,895,034, issued Jan. 23, 1990 to Poole. The disclosed powder disperser directs a gas jet at a powder sample to produce a cloud of particles. The cloud of particles passes through an annular orifice. In the annular orifice, high shear forces are applied to the aerosol transport gas, which in turn places reaction forces upon the entrained particles.
Some particles are particularly resistant to dispersion in a gas stream. These powders, known as highly cohesive powders, are typically composed of very small particles. Examples of highly cohesive powders include TiO.sub.2, CrO.sub.2, magnetic powders, polymer toners and micronized pharmaceuticals. Typically, such powders have been analyzed by suspending the powder in a liquid, called a wet dispersion process. This approach requires finding a compatible solvent and requires extra care in the disposal of contaminated solvents. Furthermore, liquids cannot be used in a time-of-flight particle size measurement system, except in the form of droplets which carry selective particle sizes.
It is desirable to provide a powder disperser which is capable of handling highly cohesive powder samples. In particular, the powder disperser should deagglomerate the powder sample into its constituent particles and supply the particles at a controlled rate. The tendency for particles to stick to surfaces in the powder disperser should be minimized.